The present invention relates generally to the management of mobile terminals in a mobile communication network and, more particularly, to a method for providing information to the mobile communication network about the position of mobile terminals.
In a mobile communication network, service areas are typically divided into a plurality of cells, with each cell served by a base station. Mobile terminals within a particular cell communicate over RF channels with the base station serving that cell. The base station may handle a plurality of simultaneous calls from mobile terminals. The base stations are connected with one another and to the Public Switched Telephone Network (PSTN) by a mobile services switching center (MSC). The MSC coordinates the activities of all the base stations and connects the mobile communication network to the PSTN. A typical MSC may handle 100,000 cellular subscribers and 5,000 simultaneous conversations at a time. The MSC also accommodates billing and system maintenance functions. In some higher density networks, several MSCs are used in a single network.
During the course of a call, the mobile terminal may move from one cell into another. A switching technique called a handover enables the call to proceed uninterrupted when the user moves between cells. When the mobile terminal moves into a different cell while a call is in progress, the MSC automatically transfers the call from the current channel being used to a new channel belonging to the base station serving the new cell.
Handover decisions are typically based on received radio signal strength and channel quality as monitored by the base station serving the mobile terminal. Received signal strengths are easily monitored and signal quality for any given channel is often determined by the bit error rate (BER) over a given channel. A handover from the current channel to another channel is initiated when the signal level or channel quality drops below acceptable levels and another channel is available that is capable of providing acceptable communications.
In digital mobile communication networks, it is common for the base stations to enlist the assistance of the mobile terminal to determine when a handover is required. In order to use a mobile terminal to assist handover, the serving base station downloads a list of channels, commonly referred to as a neighbor list, at the start of a call or after a handover. The neighbor list identifies channels in neighboring cells that are potential handover targets. In a mobile assisted handover (MAHO), each mobile terminal measures the received power from surrounding base stations identified by the neighbor list and continually reports the results of these measurements to the serving base station. The signal strength measurements are made by the mobile terminal in-between periods of communication during a call. For example, in Global System for Mobile Communications (GSM) systems, the frequency is divided into eight time slots. The mobile terminal is allocated one time slot for transmissions and another time slot to receive signals from the base station. During the remaining time slots, the mobile terminal is not communicating with the serving base station. The mobile terminal monitors channels belonging to neighboring base stations during these idle periods and then quickly returns to its assigned channels in time to transmit and receive signals in its allocated time slots. The measurements made by the mobile terminal of signals received from neighboring cells are reported back to the serving base station in a scheduled manner so as not to interfere with voice or data transmissions.
The measurement reports provided by the mobile terminal give the base station a list of the signal strength and possibly bit error rates from adjacent cells, as measured by the mobile terminal at its present location. The mobile communication network also knows which adjacent cells have unused radio channels that are available for allocation during a handover. From the list of available channels, the mobile communication network selects the cell which can best serve the mobile terminal and minimize interference. A suitable traffic channel in that cell is assigned as the target, and the mobile terminal is commanded to retune to the traffic channel in the target cell. At the same time, the call is switched by the MSC from the base station currently serving the mobile terminal to the base station in the target cell. The mobile terminal tunes to the newly assigned channel during one of the idle periods so there is no interruption in transmission. Thus, from the user""s perspective, the handover can be made seamless.
In addition, many current mobile terminals include a positioning receiver, such as a Global Positioning System (GPS) receiver, for determining the location of the mobile terminal. In the existing art, the location of the mobile terminal, as derived from the positioning receiver, by mobile terminal and is not communicated to the mobile communication network. However, if the mobile communication network was provided with the location of the mobile terminal, the mobile communication network could use this information for a variety of purposes, such as to optimize handovers. Unfortunately, the present state of the art does not provide a sufficiently convenient and efficient method of conveying position information from a mobile terminal to the mobile communication network.
Thus, there remains a need for a convenient and efficient method of reporting the position of a mobile terminal to the mobile communication network, preferably one that minimizes bandwidth needed for such signaling. Also, for packet data communications, such a method should minimize the number of access events for transmissions by the mobile terminals to report their respective positions.
The present invention is a position reporting method implemented by a mobile terminal in a mobile communication network. The mobile terminal is equipped with a Global Positioning System (GPS) receiver or other position estimating device that enables the mobile terminal to determine its current location within a mobile communication network. The mobile terminal periodically generates an estimate of its current location, referred to herein as a position estimate, and transmits the position estimate to the mobile communication network.
In one embodiment of the invention, the position estimates are combined with control signals otherwise being transmitted from the mobile terminal to the mobile communication network. For example, in digital mobile communication networks, such as GSM systems, the mobile terminal periodically sends channel quality measurement (CQM) reports to the mobile communication network while the mobile terminal is engaged in a call. The CQM reports may include signal strength and channel quality measurements, such as BER, for channels in neighboring cells that are being monitored by the mobile terminal. According to the present invention, position information can be appended to or combined with the CQM report, or other reports sent on a regular basis, to minimize signaling overhead. Similarly, when the mobile terminal is engaged in a packet data session, position reports can be time multiplexed with packet data transmitted from the mobile terminal to the mobile communication network. By multiplexing the position information with packet data otherwise being transmitted, the number of access events is reduced and collisions resulting in access attempt failures forcing retransmissions are avoided.
The position reports may be sent by the mobile terminal on a regular and unsolicited basis. Alternatively, some predetermined decision or reporting criteria can be employed to trigger the sending of a position report. For example, the mobile terminal can be programmed to transmit a position report when the difference between the current position of the mobile terminal and a previously reported position exceeds a predetermined threshold. The frequency of reporting can then be varied by the network operator by specifying the distance between reports or an explicit transmission frequency.
In another aspect of the invention, position reports may also include other information derived from position, such as the velocity, speed, direction or acceleration of the mobile terminal. A velocity estimate may be sent by the mobile terminal when the change in velocity of the mobile terminal exceeds a predetermined threshold or other reporting criteria. The velocity of the mobile terminal can be used to determine the reporting interval between consecutive position reports by the mobile communication network or by the mobile terminal. By using the velocity to determine the interval between position reports, a mobile terminal traveling at a higher speed will send position reports more frequently than a mobile terminal traveling at a lesser speed. The reporting interval can be determined by either the mobile terminal or by the mobile communication network. Velocity, speed and/or direction can also be used by the mobile communication network to interpolate the position of the mobile terminal between position reports.
The position reports transmitted by the mobile terminal may be in one of several formats. One type of position report would give an absolute position (e.g., such as a coordinate position), that can be interpreted by the mobile communication network without resort to previously-received data. Alternatively, the position report may give a relative position. A relative position is a position expressed as a function of a previously-reported position. The referenced position referred to by a relative position may, itself, be a relative position. Another type of position report is a xe2x80x9cno-changexe2x80x9d report. A no-change report indicates to the mobile communication network that the mobile terminal has not changed position beyond predefined limits. An indication of xe2x80x9cno-changexe2x80x9d can be given by omitting position data that otherwise would be transmitted in a message. Lastly, position of the mobile terminal can be implied by formatting data otherwise being transmitted in a predetermined manner. One example of implied position is to transmit channel quality measurements in order of distance. The order of the measurements would then give some indication of the relative location of the mobile terminal.